Developer Tools — landing page (solutions/tools/index)

**Page:** `solutions/tools/index.mdx`

**Context:** Many written siblings (`hiero-cli/`, `playground`, `code-repo`, `custodians-library`, redirects).

**Scope:** Landing cataloguing open-source Hedera tools; route into Hiero CLI, Playground, libraries.

### Acceptance criteria
- [ ] Replace the placeholder template with a real landing page
- [ ] Remove `hidden: true` from the frontmatter
- [ ] **No "Browse by section" block** — the side nav covers that; open straight into the deep-dive sections
- [ ] Route into the written sibling pages via ``/``
- [ ] Follow the style guide (markdown tables only; no AI-tell phrasing)